Methods and apparatus for making a substrate or applying a coating over a substrate (i.e., web coating) are employed in the manufacture of a wide variety of goods. In one example, substrate coating is commonly used in the manufacture of photothermographic, photographic, and thermographic film. Other types of goods which are manufactured by a web coating process include: pressure sensitive adhesive tapes, coated paper, magnetic audio, video and storage tapes, as well as other products.
The production of high quality articles, particularly photothermographic, photographic, and thermographic articles, consists of applying a thin film of a coating solution onto a continuously moving substrate or web. This film can be applied using a variety of techniques including dip coating, forward and reverse roll coating, gravure coating, wire wound rod coating, blade coating, slot coating, slide coating, and curtain coating. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be in the form of a succession of discreet sheets.
In some instances, the coating on the substrate will tend to streak as a result of problems during the coating process. This non-uniform streaking of the coating material will often render portions of the coated substrate unusable. Since the coating process is a very high-speed process, it becomes imperative to detect any streaks in the coated substrate as soon as possible in order to minimize the amount of unusable product. By performing an inspection at the end of the manufacturing process, as much as 1000 linear feet (or more) of defective product may already be manufactured by the time the defect is discovered.
Detecting any non-uniformities in the coated substrate during the manufacturing process can be very difficult. In the case of the manufacture of photosensitive coated substrates (e.g., photothermographic and photographic film), the manufacturing process is preformed under safe light conditions, where a relatively clear, somewhat diffusing coating is applied to a clear substrate. These safe light conditions make any sort of visual inspection very difficult.